1. Field of The Invention
This invention is directed toward the disposal of solid and liquid waste material, and more particularly to the disposal of solid and liquid waste material in a subpressured, underground earth formation which has been penetrated by a borehole.
2. Background of the Art
In disposing solid waste, the two major categories are surface or near surface disposal, and subsurface disposal. Surface disposal includes landfills or land farming operations, and subsurface disposal herein applies to underground disposal utilizing injection wells. In using either of these methods, one of the main concerns is the protection of underground sources of drinking water (USDW).
Solid waste disposal into landfills requires that the waste be of a certain moisture content and that it be sufficiently stabilized to pass testing procedures for leachibility and other properties to determine any hazardous characteristics. Testing and stabilization add to the basic cost of disposal. There is also the inherent danger that any USDW will be contaminated by seepage or leakage of the disposed waste above. Land farming operations are utilized to detoxify particular kinds of solid waste, be they municipal, industrial or oil and gas industry derived waste. In any such application, the use of added nutrients, such as fertilizers, accelerates the natural process of biodegradation in order to render the waste benign. Problems have been found in specific cases with heavy metal contaminants concentrated in some of the traditional land farm waste streams, such as municipal water processing plant sludges. These do not degrade and must be bound up utilizing some sort of chemical exchange if the product is to be rendered completely benign. Both of these surface or near surface operations, when properly managed, are operated to minimize the possibility of ground water or off site contamination. However, as the ground water is still, by definition, below these operations, the possibility of future contamination, and accompanying liability, exists.
Injection well operations can be designed to inject fluids into the downhole formations directly or through fractures created or induced in the formations. Each formation has a "fracture pressure" or "threshold" at which the formation will part. The injected fluid then travels into the formation by way of the path of least resistance, namely the fracture, and the fluid bleeds off into the formation through the walls of the fracture. In slurry injection, the solids of the slurry are placed into the volume of the open fracture and are held there after slurry injection is terminated, or, alternately, the solids intermingle with the formation solids in a "dissolution" fracture scenario and likewise move out into the formation until they are held in place following a reduction in pressure when the slurry injection ceases. The volume of waste that can be disposed of in a given well using this induced fracture methodology is limited to that which can be pumped into the formation during the fracturing operation. Further waste disposal or injection in such a well is improbable due to the difficulty in reinitiating the fracture. For long term fracture, particularly with respect to the dissolution scenario, the concern is with continuing extension of the fracture or the fracture system increasing the risk of losing control of the waste, such as impacting other wells or aquifers in the vicinity of the injection well. This concern is in addition to concerns regarding the suspension and carrying of the wastes over an extended term.
In either of the above cases, the injection pressure must be great enough to overcome the formation pressure to induce formation fracturing. Due to the induced pressures required and to the fact that fractures are not considered highly controllable, regulatory agencies are somewhat averse to disposal of waste streams, suspended solids, or otherwise, into wells in which fractures have been induced in the injection formation. More specifically, there is concern that the induced fractures may rise vertically to the extent that they will penetrate to the USDW or to a conductive zone through which the contamination potential is significantly increased, or that they will extend laterally to where fractures may intersect another well bore that might allow communication with the USDW. There are two pathways of concern through which this could occur. The first pathway is through damage caused to, or existing weakness in, the seal existing in the intersected borehole. Under this scenario, the disposed waste stream would contaminate the USDW by flowing "behind casing" in the intersected borehole. The second pathway is again through the intersected borehole in the event that it is an abandoned borehole that was not plugged and abandoned properly. For these reasons, regulatory agencies administering Class II injection wells specify a maximum surface injection pressure (MASIP) capable of protecting the integrity of the disposal zone. Often, the pressure maximum at the surface can be low so that downhole pressure is below a defined maximum pressure. This is below formation fracture pressure.
Fluid can also be injected into non-fractured formations. For injection and disposal under the non-fractured scenario, it is required that the formation be able to take and transport the injected fluids and/or slurries within the existing porous matrix. If a relatively simple fluid such as produced oilfield brine is to be injected, there are innumerable Class II injection wells that are and can be permitted to handle such fluids. With increasing viscosity of the injected fluid, increasing permeability of the disposal formation is required. If solids in suspension are to be injected, the requirement is that the formation and pore size distribution is such that the solids can enter or can be forced into the matrix. Higher injection pressures would be required to force the solids into the formation. But when considering the protection of the USDW, it is required that the injection pressure be limited to that which can be shown not to induce fracturing in the disposal zone. This would activate the previously mentioned concerns about fractured wells and fractures traveling to other zones or well bores. It would also severely limit the allowable size of the suspended waste particles for typical injection into sandstone or limestone disposal zones.
Earth formations do exist in which the permeability and porosity are high enough to allow transport of slurried solids. In most cases, the injection pressure required is still a problem due to the viscosity of such slurries. The problem of downhole injection pressure and the disposal of slurried waste can be reduced, however, if the target zone is underpressured relative to the existing hydrostatic gradient.
Attention is now directed to a particular type of waste, namely, waste generated in the drilling and producing of oil and gas wells. This type waste derives from a variety of typical oil field sources. This type of waste will be used to examine further the background of the art. The process used in the drilling of most oil and gas wells involves the use of a drilling fluid commonly referred to as drilling "mud" in the industry. The mud is injected under pressure through the drill string during drilling and returns to the surface through the drill string-borehole annulus. The mud performs multiple functions which include cooling of the drill bit, lubrication of the drill bit, providing a means of returning the drill cuttings to the surface of the earth and providing hydrostatic pressure to prevent the "blowout" of high pressure geologic zones when such zones are penetrated by the drill bit. Drilling mud comprises a liquid phase and a suspended solid phase. The liquid phase can be either fresh or saline water or even an oil base. The solid phase, which is suspended within the liquid phase, can comprise a multitude of materials blended to meet the particular needs at hand. As an example, barite (barium sulfate), with a specific gravity over 4.0, is often used as a weighting constituent to increase the bulk density of the mud when high pressure formations are being penetrated. Other additives are used to control drilling fluid circulation loss when certain types of high porosity, low pressure formations are penetrated. Once returned to the surface, the drilling fluid contains cuttings from the drill bit. Although most large cuttings are removed at the surface prior to recirculating the mud, smaller sized particles remain suspended within the drilling mud. Upon completion of the drilling operation, the drilling mud can sometimes be reconditioned and used again. Eventually, however, the mud can no longer be reprocessed and becomes classified as a waste product of the drilling operation. The waste can be "pure" waste in the sense it is mostly oil field drilling fluids exemplified above. The waste can be diluted by adding other waste streams to it.
Once the well has been successfully drilled and cased, hydrocarbons are extracted or produced from one or more formations penetrated by the borehole. Although hydrocarbons are the primary production fluids of interest, other nonhazardous oilfield waste (NOW) is usually generated in the production of hydrocarbons. A water component is usually produced along with the hydrocarbon component, and in most areas of the world, the produced waters are saline. Although there are some secondary uses for produced waters, these waters are in general considered a waste product of the production operation. Solid wastes including sand, paraffin, sludges and other solid materials are also generated during the production operations. Large quantities of these solid wastes have been accumulated for decades in production pits. Environmental regulations have led to the need for disposal solutions for the materials contained in production pits undergoing remediation to acceptable environmental levels.
The isotopes uranium-238 and thorium-232, and the radioactive isotopes associated with the decay series of these isotopes, occur in nature in earth formations. In situ, the activities associated with these decay chains are relatively low and do not present a radiation hazard during the drilling operation. During well production, however, these naturally occurring radioactive materials (NORM) are dissolved in the produced waters and are transported to the surface. Over an extended period of time, the NORM becomes concentrated in precipitated scale associated with tubulars and surface equipment such as heater treaters, wellheads, separators and salt water tanks. Although the parent isotopes uranium-238 and thorium-232 are not generally present, the decay products or "daughter" products radium-226, radium-228, radon-222 and lead-210 can be found in oilfield waste. Radium-226, which coprecipitates with carbonates and sulfates of calcium, barium and strontium, is by far the greatest source of radioactive waste resulting from production activities. Once atoms of radium have replaced a sufficient number of atoms of the elements normally found in NOW waste to exceed a specified regulatory level, the waste is classified as NORM. Stated another way, there is no difference between NOW and NORM waste other than the level of radioactivity, which usually results from the radium content of NORM waste.
In summary, the drilling and production of oil and gas wells generates much waste. The wastes are classified as nonhazardous oilfield waste (NOW) and naturally occurring radioactive materials (NORM). NOW originating from drilling and production operations is primarily composed of drill cuttings, sand and spent material such as drilling mud which is no longer suitable for use and must be managed as waste under regulatory authority. Such mud might contain salts, non-toxic metals such as sodium and calcium, toxic metals such as barium, chromium, lead, zinc and cadmium, and oil and grease contamination from the introduction of diesel oil (oil based mud), crude oil or a multitude of hydrocarbon based additives. The spent mud, with associated contaminants, comprises a liquid and a solid phase. NOW is also generated in production operations where copious amounts of saline water, along with some solids (sand), may be produced with the desired hydrocarbons. NORM originates primarily from production operations wherein the previously described radioactive scale contaminates not only large pieces of hardware such as well heads and separators but also can contaminate produced "waste" fluids such as salt water and associated solids. It is necessary to dispose of all types of waste, including those previously stored in pits, in a manner which will not contaminate the surface of the earth and not contaminate subterranean aquifers used as sources of drinking water.
As discussed previously, there are surface or near surface disposal means, and subsurface disposal means for the disposal of both NORM and NOW material. Other oil field wastes are often stored, either temporarily or longer, in such pits or portable containers.
Present surface and near surface disposal means will first be examined. Oil and grease toxicity in NOW can be lowered by dilution techniques. Organics can be converted biologically to less toxic forms. Organics can also be removed by extraction processes. These extraction processes can utilize heat and may include methods such as thermal desorption or incineration. Oils can be removed by separation techniques and possibly produce a byproduct of commercial value. Organics can also be bound to solids thereby reducing their leachability and hazard to drinking water supplies. Salts can be diluted and discharged, chemically destroyed or rendered insoluble. Heavy metals can neither be biologically or chemically changed into less toxic species, therefore dilution with non-contaminated materials is one method of controlling possible hazardous pollution. Heavy metals can be bound chemically thereby rendering them immobile and nonleachable into the environment. NORM can not be destroyed or chemically altered, therefore dilution with essentially non-radioactive material to prescribed levels is an acceptable method. All of the above steps are necessary if the waste material is to be disposed of using present surface or near surface disposal means. It is apparent that such "preprocessing" can add significantly to the costs of disposal and, as mentioned previously, underlying USDW can still be at risk.
If present subsurface injection means of disposal are employed, numerous criteria discussed previously must be met. Among these criteria are defined geologic conditions surrounding the injection well, proper casing and cementing of wells penetrating the injection zone, a MASIP, and specific procedures for periodic testing and reporting to various regulating agencies. Sometimes, good (meaning conventional) engineering practices will define the pipe schedule, number of strings, amount of casing, etc.; sometimes, the casing and pipe requirements are defined by regulations of the states. These vary dependent on regulation. Furthermore, current injection technology requires that the particle size of the solid phase of any slurry first be minimized before injection. This is to prevent clogging or "sanding" of the perforations opposite the injection zone and also to prevent the filling of pore space throats of the injection zone thereby reducing permeability. Processing time and cost must be incurred, and the large particle size solid component of the slurry must still be disposed of in an environmentally acceptable manner. The meeting of all of the above criteria greatly increases the cost and time required to dispose of oilfield waste in underground disposal formations.
An objective of the present invention is to provide means for disposing of waste material in underground formations penetrated by a borehole. A further objective of the invention is to provide means for disposing of slurried waste in an underground formation. A still further objective of the invention is to permit the disposal of waste fluid within a disposal formation by injecting the fluid at a pressure below the fracture pressure of the formation. A further object of the invention is to permit disposal of a waste fluid within a disposal formation by injecting the fluid at a pressure below the MASIP. A still further object of the invention is to establish selection criterion for locating subpressured or underpressured disposal formations in which waste fluids can be flowed under the force of gravity. The selection criteria assures that the downhole injection pressure does not exceed the disposal formation fracture pressure. Low pressure surface operation coupled with down hole pressures below fracture enable long term use to inject a great variety of waste materials. A yet further object of the invention is to provide means and methods for pretreating waste slurries such that they can be optimally flowed or pumped into underground disposal formations. There are other objects of the invention which will become apparent in the following disclosure.